SPL is a well known waste resulting from the aging of used electrolytic cell cathodes in primary aluminium production. This cathode is formed from refractory material, conductive carbonaceous material and other various materials (iron bars, insulators, etc.). Over the operating time, these materials react with the content of the tanks and effectiveness in their operation is lost, and they must be disassembled.
A toxic and hazardous waste, SPL, is thus obtained which is normally poured in to special dumps, as such, or after some impurities have been removed. This waste has several characteristics, depending on the cut made when disassembling the lining from the used cathodes: refractory, carbon or mixed. CN−, nitrides, carbides, fluorine, etc, are the most important impurities.
Due to the toxicity of this waste, the government must control the treatment and destination of this material in order to not affect the environment. The carbonaceous part has the most evident application for power production, but must also be prepared.
SPL is generally treated in dry or wet conditions to destroy the most harmful impurities, obtain an inert waste, or separate and reuse any of its present constituents.
To date, there are many different processes for inserting, recycling and reusing the material from these cathodes, however, most of them are focused on reducing or preventing the hazard involved in handling the powdery waste, and are focused on adding some type of substance with passivating properties reducing the hazard.
In this same sense, U.S. Pat. No. 5,352,419 describes a process for the recovery of aluminium and fluorides from SPL comprising the steps of calcining SPL to produce an ash with environmentally acceptable levels; the ashes are subsequently filtered in a solution containing a mineral acid, obtaining the corresponding aluminum salt in a sufficient amount to dissolve the aluminum and fluoride contents, in order to subsequently subject the filtered liquid to hydrolysis to cause precipitation of an aluminum fluoride product.
European patent EP0550136 describes a method for the treatment of SPL, including refractory materials, to transform SPL into a state in which it can be used as a filler material or as raw material. The SPL is crushed and supplied to an electrothermic smelting furnace, optionally together with a SiO2 source, in which the SPL is melted at a temperature between 1300 and 1750° C. An oxidizing agent is supplied to the melt to oxidize carbon and other oxidizable components contained in the SPL such as metals, nitrates and carbides. A calcium oxide source is also supplied to the smelting furnace in the amount necessary to react with all the fluorides present in the form of CaF2, the slag of which is liquid at the bath temperature inside the furnace. The calcium aluminate slags, or the calcium aluminate silicate slags and, optionally a metal phase, are used for blocks and granules.
U.S. Pat. No. 6,132,574 relates to a bottom lining for electrolytic cells, especially for electrolytic cells of aluminium from primary production and from secondary refining, the base body of the electrolytic cells being formed by steel pots, as well as a process for manufacturing it.
All the previous patents have attempted to recover certain substances contained in SPL and produce an inert waste which can be poured without problems. To that end, pilot and industrial plants have been built to carry out the operations for that purpose. However, the investments to be made are very large, the process for treating SPL was very complex, the waste obtained was not as inert as expected, and the products obtained did not have the desired quality. It is therefore necessary to develop new processes or routes for eliminating or treating SPL with lower associated costs and in which good results as regards inerting and clarity are obtained in order to affect the environment to the lowest extent possible.